A marine propulsion apparatus, also known as an outboard motor, is a heavy engine to be disposed above a screw. The stern is always sunk deeply into the water when such a marine propulsion apparatus is mounted on the stern of a hull. The sinking is reduced when the boat is running at a certain velocity. However, water resistance is considerable, time is required to increase the velocity, and smooth acceleration is difficult to obtain because the depth of the stern in the water is considerable when the boat is accelerating from a stopped state to a running state.
A lift generation plate is effective as a countermeasure to this problem. A lift generation plate is disclosed in, e.g., Japanese Patent Application Laying-Open Publication No. 57-60995 (JP 57-060995 A). This lift generation plate will be described with reference to FIGS. 36, 37 hereof.
An outboard motor 100 is provided with an anti-cavitation plate 103 in the upper section of a casing 102 that supports a propeller 101, as shown in FIG. 36. A splash plate 104 is provided above the anti-cavitation plate 103, and a lift generation plate 105 is disposed above the splash plate 104.
The lift generation plate 105 is a flat plate in which a large concave portion 106 is opened in the center and to which stays 107, 107 are provided at the front end, as shown in FIG. 37. The casing 102 indicated by the imaginary lines is inserted into the concave portion 106 in a relative manner, whereby the lift generation plate 105 is mounted on the casing 102 in the manner shown in FIG. 36. As a result, a dead space 109 indicated by the diagonal lines in FIG. 37 is unavoidably produced. The dead space 109 does not contribute in any way to the generation of lift. The lift obtained by the lift generation plate 105 is therefore reduced.
A structure that can take the place of the structure described above is disclosed in U.S. Pat. No. 4,756,265. This structure will be described with reference to FIG. 38, wherein a lift generation plate 114 is hung on an arm part 113 that extends to the left in the Figure from a casing 112 that supports a propeller 111. In other words, the lift generation plate 114 is disposed near the propeller 111. A vortex is generated when the propeller 111 is rotated at high speed. The lift generation plate 114 is exposed to the vortex. The resulting lift fluctuates because the vortex flow is turbulence. The lift generation plate 114 is moved in the depth direction of the Figure in order to avoid the effect of the vortex. The lift generation plate 114 decreases in size in conjunction with this movement, and the resulting lift is reduced.
In view of the above, there is a need to devise a lift generation plate in which the resulting lift is considerable.